In a traveling type hydraulic machine such as a hydraulic shovel, a traveler (e.g., crawler) to be driven by a traveling motor is overlaid by a swivel slide to be swiveled by a swiveling motor. To this swivel slide, there is mounted a boom to be actuated by a boom cylinder. To this boom, there is attached an arm to be actuated by an arm cylinder. To this arm, there is attached a bucket to be actuated by a bucket cylinder.
As a hydraulic control system for such traveling hydraulic machine, there has been frequently adopted a two-pump system which uses two hydraulic pumps as its oil pressure source. Specifically, the first hydraulic pump is connected to a first direction change-over valve group composed of a plurality of change-over valves, and the second hydraulic pump is connected to a second direction change-over valve group composed of a plurality of direction change-over valves. Generally speaking, for example, a traveling righthand change-over valve, a bucket change-over valve and a boom I change-over valve belong to the first direction change-over valve group, and a traveling lefthand change-over valve, a swiveling change-over valve, an arm change-over valve and a boom II change-over valve belong to the second direction change-over valve group. Moreover, these change-over valves are generally constructed as stacked type control valves.
The performance required of these control valves is that a necessary amount of oil under pressure be fed to righthand and lefthand traveling motors when at least one of the hydraulic actuators of the aforementioned front working machines (e.g., the boom, the arm, the bucket or the swivel slide) is operated simultaneously with the traveling motor. This performance is necessary for ensuring the excellent forward travel of the traveler. From the aspect of safety, moreover, a sufficient amount of oil is desirably fed to a swivel actuator when this actuator is operated during the travel. It is also desired as well as the above-specified performance that the stacked state be small-sized and compact to require no large space for mounting and that the piping can be accomplished simply at a reasonable cost. However, the prior art has failed to provide a practical control valve capable of satisfying the above-specified desires.
Specifically, the control valve for controlling the hydraulic shovel is disclosed in Japanese Patent Laid-Open Nos. 263710/1985, 83405/1988, 34304/1988 and 219905/1988.
However, these prior art technologies are directed to a hydraulic circuit mainly for controlling the simultaneous operations of the arm and the swivel. Specifically, one direction change-over valve group is equipped with a merging direction change-over valve, which is operated to feed the arm direction change-over valve with a merging oil under pressure coming from the first hydraulic pump and the second hydraulic pump. Therefore, the hydraulic circuit cannot solve the problem coming from the simultaneous operations of the traveling motor and another actuator. In case the front working machine such as the arm is moved during the travel, the more oil under pressure will flow into the arm cylinder under the lighter load. As a result, the feed of the oil under pressure to the traveling motor belonging to the group shared with the arm cylinder becomes short so that the problem of curved travel has failed to be solved.
For this solution, it is necessary to establish the communication between the traveling two direction change-over valves. This communicating mechanism according to the prior art cannot be provided without equipping the prime mover with external pipings or electromagnetic valves. These equipments have raised the cost and caused another problem of large space for mounting the valves.
From the composite operability of the swivel and arm, on the other hand, it is desirable at the time of the composite operations of the arm lowering and swiveling operations to feed sufficient oil under pressure to the swiveling motor of a higher load while restricting the inflow of the oil to the arm cylinder of a lighter load, it is also desired at the time of the composite operations of the arm lifting and swiveling operations to feed the oil under pressure to the arm cylinder from parallel circuits not through any throttle. Despite this fact, however, the prior art has failed to provide a compact and practical valve structure for realizing that function.